1. Field of the Invention
The present invention relates generally to wireless communication systems and, in particular, to handoff in wireless communication systems.
2. Description of the Related Art
The operation of wireless communication systems is in large part defined by industry standards. These standards specify the air interface specification that allows a mobile terminal to communicate with a base station. The air interface specification typically includes a set of air interface channels, channel signal encoding rules, and signaling messages to allow the mobile terminal to communicate with the base station. The standards include standards for Advanced Mobile Phone Service (AMPS), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Global System for Mobile (GSM) technologies. The rapid pace of development in wireless communication systems has typically led to multiple standards within some of the technologies, such as CDMA. For example, some of the standards in CDMA technology include IS-95, CDMA 2000, wideband CDMA, and Universal Mobile Telecommunication System (UMTS).
A particular wireless communication system will comply with one, or possibly several, standards. For example, many wireless communication systems that comply with either one of the CDMA standards or one of the TDMA standards are designed to also comply with the AMPS standards. Systems that comply with more than one standard maximize geographic coverage for users since these systems allow a user who is capable of using either standard to use the wireless communication system.
A wireless communication system usually serves a specific limited geographic area and operates over a particular frequency band. The frequency band is assigned by the FCC for a particular geographic area. The geographic area served by one wireless communication system will typically border on, or overlap, with the geographic area served by another wireless communication system. In the case of adjacent wireless communication systems, the systems may operate either over the same or over different frequency bands, and may comply with either the same or with different standards. In the case of two overlapping wireless communication systems, the systems will operate over different frequency bands, and may comply with the either the same or with different standards.
FIG. 1 illustrates two wireless communication systems, wireless communication system 100, and wireless communication system 200. The geographic areas serviced by wireless communication systems 100 and 200 are divided into a plurality of spatially distinct areas called xe2x80x9ccells,xe2x80x9d 102 and 104, and 202, respectively. Although cells 102, 104, and 202 are illustrated as a hexagon in a honeycomb pattern, each cell is actually of an irregular shape that depends on the topography of the terrain surrounding the cell. Although there are only two cell shown for wireless communication system 100 and one cell shown for wireless communication system 200, one skilled in the art will appreciate that each wireless communication system typically has a much larger number of cells. Each cell 102, 104, 202 contains one base station 112, 114, and 212, respectively. Base stations 112 and 114 include equipment to communicate with Mobile Switching Center (xe2x80x9cMSCxe2x80x9d) 120, which is connected to local and/or long-distance transmission network 122, such as a public switch telephone network (PSTN). Base station 212 includes equipment to communicate with Mobile Switching Center (xe2x80x9cMSCxe2x80x9d) 220, which is connected to local and/or long-distance transmission network 222, such as a public switch telephone network (PSTN). Each base station 112, 114, and 212 also includes antennas and radios, which include a receiver and transmitter. The base stations use the antennas and radios to communicate with mobile terminals 124, 126.
When a call is set up in wireless communication system 100, mobile terminal 124 communicates with the base station from which mobile terminal 124 receives the strongest signal, in this case base station 112. As mobile terminal 124 moves from one cell to another, continuous service is provided by xe2x80x9chanding offxe2x80x9d mobile terminal 124 from the base station in cell where the mobile terminal is located to the base station in the cell into which the mobile terminal is moving. Based on measurements of signals sent to and received from mobile terminal 124, base station 112 determines the best base station with which it can participate in a handoff of mobile terminal 124, referred to herein as a handoff candidate. The receiver and transmitter of the handoff candidate are referred to herein as the candidate receiver and the candidate transmitter, respectively. In CDMA systems, base station 112 starts the process of searching for a handoff candidate when the signal received by mobile terminal 124 from base station 112 falls below a certain threshold. The mobile terminal is continuously measuring the pilot signals from base stations in adjacent cells. Base station 112 then analyzes these signal measurements to determine the handoff candidate. In AMPS and in some TDMA systems, base station 112 starts the process of searching for a handoff candidate when the signal received by the base station from the mobile terminal falls below a certain threshold. Base station 112 searches for a handoff candidate by analyzing the signals from base stations in adjacent cells. The base station requests the base stations in adjacent cells to measure the signals received by them from mobile terminal 124. Base station 112 then analyzes these signals to determine the handoff candidate. In TDMA systems that have mobile assisted handoff, both the signal received by the base station from the mobile terminal and the signal received by the mobile terminal to the base station is used to determine when the base station starts the process of searching for a handoff candidate.
A problem occurs when mobile terminal 124 may need to be handed off between two wireless communication systems. For example, when base station 112 is in a cell on the edge of wireless communication system 100, such as cell 102, and wireless communication system 200 either does not operate over the same frequency band, and/or does not comply with the same standard as wireless communication system 100. This is a problem because before mobile terminal 124 can be handed off, the mobile terminal needs to communicate with a base station in wireless communication system 200 and determine the characteristics and environment in wireless communication system 200. In order to communicate with a base station in wireless system 200, mobile terminal 124 needs to switch to operate in the same frequency band as communication system 200. Mobile terminal 124 also needs to switch to comply with the standard with which wireless communication system 200 complies. The mobile terminal then needs to communicate with base station 212. Since mobile terminal 124 is not aware of the characteristics of base station 212, it takes mobile terminal 124 a significant amount of time to establish communication with base station 212. For, example, when both wireless communication systems 100 and 200 comply with a CDMA standard, mobile terminal 124 needs to: switch to the appropriate frequency for wireless communication system 200; locate and synch to the pilot signal from base station 212; and register with and be authenticated by wireless communication system 200. During this time, mobile terminal 124 cannot communicate with base station 112 because base station 112 operates in a different frequency band and/or complies with a different standard. The significant amount of time required to perform the handoff may result in the call being dropped, or in the user of mobile terminal 124 hearing noise or silence, which is a disruption of the call and an inconvenience to the user.
Accordingly, there exists a need for quickly performing a handoff with a base station in one wireless communication system and a base station in another wireless communication system.
The invention solves the above problems by using the location of a mobile terminal to direct the mobile terminal to a candidate receiver for participation in a handoff of the mobile terminal. The location of the mobile terminal in communication with a first receiver is determined. The location of the mobile terminal can be determined in any way. For example, the location of the mobile terminal can be determined using a time difference of arrival triangulation, a global position system (GPS), or a wireless assisted global position system. It is also determined whether the mobile terminal should participate in a handoff. Whether the mobile terminal should participate in the handoff can be based on the location of the mobile terminal. Alternatively, this can be based on any method of determining whether the mobile terminal should participate in the handoff, such as by measuring a signal strength of a signal received by or sent by the mobile terminal and comparing the signal strength to a threshold signal strength. When location is not used to determine whether the mobile terminal should participate in a handoff, the steps of determining the location of the mobile terminal and whether the mobile terminal should participate in a handoff can be reversed.
If the mobile terminal should participate in the handoff, the mobile terminal is directed to the candidate receiver for participation in the handoff using the location of the mobile terminal. The candidate receiver can be identified by: 1) accessing the location of the mobile terminal within the wireless communication system in an index, such as a database or a lookup table, where the index correlates the location within the wireless communication system and one or more candidate receivers for participating in the handoff with the first receiver; and 2) obtaining the candidate receiver from the index. The candidate receiver then participates in the handoff of the mobile terminal, thereby allowing the mobile terminal to communicate with the candidate receiver.
In an embodiment of the invention, directing the mobile terminal to the candidate receiver includes providing the mobile terminal at least one characteristic of the candidate receiver and/or transmitter, referred to herein as the candidate transceiver. The characteristic is preferably an RF communication characteristic that enables communication between the mobile terminal and the candidate receiver and candidate transmitter.
In one embodiment of the invention, the mobile terminal then performs measurements of the wireless communication system using the candidate receiver. The first receiver then participates in a handoff of the mobile terminal. An optimal receiver for participation in the handoff is identified based on the measurements of the wireless communication system that the mobile terminal obtained when it was communicating with the candidate receiver. The optimal receiver can be identified by either the mobile terminal or the base station containing the first receiver. The optimal receiver then participates in a handoff of the mobile terminal.
Identifying the candidate receiver for participation in the handoff using the location of the mobile terminal is particularly advantageous when: 1) the first receiver operates with a first frequency band, and the cell, in which the first receiver is located, borders at least one other cell having a receiver that operates at a second frequency band; 2) the first receiver complies with a first wireless communication standard, and the cell, in which the first receiver is located, borders at least one other cell having a receiver that complies with a second wireless communication standard; or 3) both one and two. For example, when the first receiver complies with a CDMA standard and the candidate receiver complies with AMPS, or TDMA, or GSM; when the first receiver complies with a GSM standard and the candidate receiver complies with AMPS or TDMA; when the first receiver complies with IS-95 CDMA and the candidate receiver complies with a wideband CDMA standard; or when the first receiver operates within frequency band A and the candidate receiver operates within frequency band B.